Neurobehavioral Consequences of Aging and Chronic Methylmercury Exposure: Interactions with Dietary Selenium

Abstract

Methylmercury (MeHg), although a well-described toxicant to the adult and developing nervous system, is inadequately understood in adult onset, low-level chronic exposure. Among the known effects are somatosensory deficits and weakness in the extremities. Selenium (Se), an essential nutrient, might allay chronic neurotoxic effects of MeHg. To examine this, sixty-day-old female Long Evans rats were fed a diet containing 0.06 or 0.6 ppm of Se as sodium selenite; both diets providing adequate Se. After 100 days, methylmercuric chloride was introduced into their drinking water in concentrations of 0.0, 0.5, 5.0, or 15.0ppm. Both exposures continued daily for 16 months. Approximately every two months, forelimb grip strength, tactile sensitivity in the tail, and overnight running distance were tested. Flexion and hindlimb cross (clasping reflex) were also examined on a regular basis. Age-related changes were noted in control groups on grip strength, tail sensitivity, and running. MeHg-related effects were observed only in the 5.0 and 15.0 ppm exposure groups. The severity and latency to effects were influenced by MeHg dose and dietary selenium. In unexposed animals running increased with age, the increase being larger in the high-Se animals. The 5.0 ppm exposure groups showed a smaller increase in running, and Se influenced this effect. Running during young adulthood was a good predictor of running in older rats for unexposed animals. Age-related correlations were also noted on other measures. MeHg exposure generally weakened or eliminated these correlations, even in the 0.5 ppm exposure group for which no other signs appeared. Flexion always appeared before hindlimb cross, and forelimb grip strength together with tail sensitivity were both delayed by selenium in the higher methylmercury groups. Dietary selenium afforded a degree of protection against MeHg's effects, largely by delaying their onset.